1. Field of Invention
This invention relates to systems and methods for storing information.
2. Description of Related Art
Tape storage is often used as an inexpensive backup for on-line storage, increasing the reliability of computer-stored data by providing a redundant storage location. Additionally, hierarchical storage management (HSM) systems use tape storage to greatly expand the capacity of a fixed disk-based file system. Files are migrated from the disk-resident file system to tape storage when the disk-resident file system runs out of space, and files are migrated from tape to fixed disk when they are referenced. Most files in an HSM system are stored only on tape, and no redundant copy is stored on disk.
One method of storing backup information in an HSM system is to store two copies of the information, i.e., data mirroring. This way, stored information can be reconstructed even if a primary and one backup information source are damaged or lost.
Another method for storing backup information is the Redundant Arrays of Inexpensive Tapes (RAIT) technology. In a RAIT system, a collection of N+1 tapes are aggregated to act as a single virtual tape. In a typical implementation, data files are simultaneously written to blocks of the N tapes in stripes, and the parity information, which is the bit-wise exclusive-OR of data written in the blocks, is stored on the additional tape drive simultaneous with storing the data files on the N tapes. The RAIT system has a higher performance compared to systems that store duplicate copies of information because writing data in parallel to multiple tapes results in high speed storage. However, because data is stored in stripes across multiple tapes in the RAIT system, all of the tapes in a RAIT stripe, i.e., a group of tapes storing a particular set of data, must be mounted and read synchronously to reconstruct a file stored on the tapes. Because data must be synchronously read from tapes in the RAIT stripe, special hardware, or software emulation, for reading the tapes is typically required, and if one of the tape drives is not operating properly, data cannot be properly read from any of the tapes. That is, the system must wait until all of the tapes and associated tape drives are operating properly before any data can be read from the tapes.